Intracavity adaptive correction of an unstable standing-wave resonator based on reconstruction matrix optimization.

Due to the existence of the expanding beam portion in the positive branch confocal unstable resonator, the laser passes through the intracavity deformable mirror (DM) twice with different apertures, which makes it complicated to calculate the required compensation surface of the DM. In this paper, an adaptive compensation method for intracavity aberrations based on reconstruction matrix optimization is proposed to solve this problem. A collimated probe laser of 976 nm and a Shack-Hartmann wavefront sensor (SHWFS) are introduced from the outside of the resonator to detect intracavity aberrations. The feasibility and effectiveness of this method are verified by numerical simulations and the passive resonator testbed system. By adopting the optimized reconstruction matrix, the control voltages of the intracavity DM can be directly calculated from the SHWFS slopes. After compensation by the intracavity DM, the beam quality ß of the annular beam coupled out from the scraper is improved from 6.2 times diffraction limit to 1.6 times diffraction limit.

24.6 kW near diffraction limit quasi-continuous-wave Nd:YAG slab laser based on a stable-unstable hybrid cavity.

A 24.6 kW quasi-continuous-wave (QCW) Nd-doped yttrium aluminum garnet (Nd:YAG) slab laser is proposed in this Letter. The laser is based on a stable-unstable hybrid cavity. A stable and an unstable resonator were constructed along orthogonal directions in the aperture of the slab. Due to features of the hybrid cavity, the slab laser achieves both high efficiency of power extraction with excellent beam quality and compactness with simplicity. Average output power of 24.6 kW with 47% optical-to-optical efficiency is achieved in the experiment. The beam quality of the output beam is 1.5 times diffraction limits after correction of adaptive optics. The repetition frequency and pulse width of the laser are 400 Hz and 200 µs.

Vibration identification based on Levenberg-Marquardt optimization for mitigation in adaptive optics systems.

When high performance is expected, vibrations are becoming a burning issue in adaptive optics systems. For mitigation of these vibrations, in this paper, we propose a method to identify the vibration model. The nonlinear least squares algorithm named the Levenberg-Marquardt method is adapted to acquire the model parameters. The experimental validation of the high performance of vibration mitigation associated with our identification method has been accomplished. Benefiting from this method, vibrations have been significantly suppressed using linear quadratic Gaussian control, where the root-mean-square of the residual vibrations has been reduced down to a portion of a microradian. Moreover, the experimental results show that with the model identified, vibrations ranging from wide low-frequency perturbation to high-frequency vibration peaks can be dramatically mitigated, which is superior to classical control strategies.

Adaptive compensation of intracavity tilts during lasing through detecting the direction of output beams.

We present adaptive compensation of intracavity tilts based on detecting the direction of the output beams. We use the initial direction of the output beam when the laser cavity is well collimated for reference. Then the difference between the actual direction of the output beam and the reference is used as feedback to control the intracavity tip/tilt mirror. The relation between the direction of the output beam and the intracavity tilt is investigated with both the Fox-Li method and measurements. A series of experiments demonstrate that intracavity tilts can be well compensated by the proposed method. We have also proved that it is possible to substitute the proposed method for conventional extracavity beam stabilization.

Adaptive aberration correction of a 5 J/6.6 ns/200 Hz solid-state Nd:YAG laser.

In this Letter, we present an adaptive aberration correction system to simultaneously compensate for aberrations and reshaping the beams. A low-order aberration corrector is adapted. In this corrector, four lenses are mounted on a motorized rail, whose positions can be obtained using a ray tracing method based on the beam parameters detected by a wavefront sensor. After automatic correction, the PV value of the wavefront is reduced from 26.47 to 1.91 µm, and the beam quality ß is improved from 18.42 to 2.86 times that of the diffraction limit. After further correction with a deformable mirror, the PV value of the wavefront is less than 0.45 µm, and the beam quality is 1.64 times that of the diffraction limit. To the best of our knowledge, this is the highest performance from such a high-power, high-pulse repetition rate Nd:YAG solid-state laser ever built.

Automatic low-order aberration correction based on geometrical optics for slab lasers.

In this paper, we present a method based on geometry optics to simultaneously correct low-order aberrations and reshape the beams of slab lasers. A coaxial optical system with three lenses is adapted. The positions of the three lenses are directly calculated based on the beam parameters detected by wavefront sensors. The initial sizes of the input beams are 1.8 mm×11 mm, and peak-to-valley (PV) values of the wavefront range up to several tens of microns. After automatic correction, the dimensions may reach nearly 22 mm×22 mm as expected, and PV values of the wavefront are less than 2 µm. The effectiveness and precision of this method are verified with experiments.